—We are no more capable of doing this than we are of landing a man on the moon and returning him safely to the earth by the end of the decade...—
We already had the technology. It was a matter of putting the pieces together. Henry Ford also wanted his engineers to produce a V8 and they said it couldn’t be done. But they did it.
This is many more levels of magnitude more difficult.
IOW, it is like telling Galileo that you want him to design a system capable of landing a man on the moon and returning him safely to the earth by the end of the decade. He was really smart, but not that smart.
There were V8 (and V12 and V16) engines long before 1932. Ford’s engineers told him they couldn’t design a reliable V8 for the price he wanted, and they were right. It took several years for the Ford flathead V8 to be debugged to the point of reliability.
Personally, I doubt we could even go back to the moon these days.
He wasn't 'Jeopardy' smart.
With the advent of AI it is possible to build DNA arks piloted and crewed by robots that can go out and explore our neck of the galaxy to explore the "Goldilocks Zone" planets we are just now discovering. We will find dozens of earthlike planets - earthmass, watery, temperate - in the next few years, guaranteed.
In that time robotics and AI will have advanced to the point where we could start launching these arks with slow accel ion drives.
We might not hear back from these arks in many hundreds or even thousands of years, but within a few million years we will have seeded the planets.
We can even give the AI the prime directive so that it skips planets with life that is more evolved than, say, bacteria or single celled organisms.